Question

everyone

I wanna add a column start_date whose default value is current date(), so I use following commands:

alter table validation add column start_date date default CURDATE()

but it didn’t work and told me it has syntax error.

While using such following command works well:

alter table validation add column start_date date default 0

Any suggestion is appreciated.

Answer 1

From the online documentation (my bold):

The DEFAULT value clause in a data type specification indicates a default value for a column. With one exception, the default value must be a constant; it cannot be a function or an expression.

This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE.

The exception is that you can specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column.

That’s why default 0 works (a constant) but default curdate() doesn’t (a function).

You have (at least) a couple of possibilities to work around this.

The first is to use a timestamp column instead of a date column, and set its default to CURRENT_TIMESTAMP. Unfortunately, that means you’ll have to turn that back into a date when extracting it.

The second is to use triggers to set the column value, rather than from a default value. For example, use an insert trigger to set the relevant column to the current date.